1. Field of the Invention
The invention provides means for transferring power between two electrical utility grids.
2. Description of Related Art
It is an established practice in the generation, transmission and distribution of electric power to interconnect a plurality of polyphase power sources into a "grid" comprising a plurality of sources synchronized to have identical frequency, number of electrical phases, and electrical phase angles. Grids may be very large and may include multiple utility systems as well as a large number of co-generation sites. Within the United States there are only four such grids.
It is also known to transfer electric power from one grid to another. This is usually done by a cumbersome process of converting AC electric power at a first frequency and a first phase from the supplying grid into DC power, which is then transferred to the second grid and inverted to AC power at a second frequency and second phase. It is customary to carry out this process using well known solid state devices, such as transistors or rectifiers, in the rectification and inversion steps. The use of semiconductor devices in the prior art of intergrid transfer leads to the generation of substantial harmonic current and voltage distortion. It is conventional to overcome this problem by providing three filters (one on the DC link, and one on each AC end of the inter-grid transfer apparatus), an approach that can increase the amount of land needed for the intergrid transfer station by nearly 50%.
Various means of avoiding harmonic distortion in AC-DC electrical power conversion have been suggested. The inventor's teachings in this area have included his U.S. Pat. No. 4,870,558, and his pending U.S. patent application Ser. No. 08/287,489, the disclosures of which are herein incorporated by reference.